SPECIAL PROBLEMS OF SHORE LIFE 8i 



shore animals to variations in the medium are relatively 

 enormous. The Polyzoan Memhranipora membranacea, for 

 instance, has been found living in brackish water up to 

 one-tenth of the normal salinity. The colony growing in 

 this situation showed variations in the number and arrange- 

 ment of the cell spines and the number of tentacles ; these, 

 however, seemed to be related rather to the movement of 

 the water than to the salinity (Loppens, 1906). On the other 

 hand, animals from near or below low- tide mark, even when 

 the change is accompUshed exceedingly slowly, show far 

 less power of accommodation. Beudant, for instance, who 

 experimented as long ago as 1816 (see Fredericq, loc. cit.), 

 by gradually diluting the sea-water in which common shore 

 and off-shore forms were living, showed that forms such as 

 Balanus striatiis, Patella vulgata, Purpura lapillus, Cardtum 

 edule, Ostrcea edults, and Mytilus edulis, were able to survive 

 in water that was completely fresh, whereas Haliotis tuber- 

 culafa, Buccinum undatutn, Tellina tncarnata, Pecten vartus, 

 etc., all succumbed before the experiment was completed (see 

 Plate IX). 



While typical rock-shore forms like Balanus, Patella, 

 Purpura, etc., and estuarine forms like Cardtum, Ostrcea, 

 and Mytilus, are able to tolerate water which is completely 

 fresh, there is undoubtedly a mean optimum salinity for each 

 of these species, and it is only when exposed to this that 

 the animal is capable of reaching its full development. 

 What this optimum salinity is seems never to have been 

 determined for the majority of shore forms, but for species 

 of economic importance it has been ascertained empirically 

 to within fairly narrow limits. In many estuaries, for 

 instance, where the bed of the main channel is stony, mussels 

 may be distributed from the lowest outlet to a point a couple 

 of miles upstream, as in the river Dovey, which enters 

 Cardigan Bay. Only those mussels, however, which are 

 situated some distance up from the mouth of the estuary are 

 capable of growing to marketable size. The shells of bivalves 

 which have grown in unfavourable conditions of salinity are 

 typically stunted and often deformed. In cases of this kind 



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